1. Field of the Invention
This invention relates to vacuum and floor cleaners that are controlled by electronic controllers. In particular, it relates to such cleaners where the speed of the appliance is controlled during a starting period.
2. Description of Related Art
Vacuum cleaners largely fall into two broad categories. Firstly, those in which the whole cleaner is moved across the surface to be cleaned: such cleaners are normally referred to as "upright" cleaners. Secondly, those in which the main body of the cleaner is connected by a flexible hose to a cleaning nozzle that is moved across the surface to be cleaned: such cleaners are normally classed as "cylinder" cleaners. A variant of this second category has a centrally located, static body and a system of ducts and hoses to provide local cleaning. In each case, cleaning action is in part provided by suction that is produced by a fan unit driven by an electric motor.
Traditionally, the electric motor used in vacuum cleaners is a series-commutated motor with a wound armature and either an energizable winding or a permanent magnetic field. Such motors are well documented in the art, e.g. in "Electric Motors and Drives", Hughes, Heinemann Newnes, 1980, which is incorporated herein by reference. A typical torque vs speed characteristic of this type of motor is shown in FIG. 1, from which it can be seen that the torque is relatively high when the motor is initially connected to the supply and falls off as the speed rises. A typical torque vs speed curve for a vacuum cleaner fan is also shown in FIG. 1, showing that the load presented by the fan is low at low speed but rises rapidly with speed. The difference between the two curves represents the accelerating torque that is available at any speed to accelerate the load. Hence, it follows that the fan will accelerate rapidly when the motor is initially connected to the supply but that the acceleration will fall as the curves come together and the motor will run stably at the speed where the curves cross.
Because the accelerating torque is high at standstill, and because there is a significant inertia associated with the rotor of the motor and the fan unit, there is considerable torque reaction at start-up. While this is not so much a problem with upright cleaners (because the mechanical layout is generally such as to cope reasonably well with the reaction torque), it is more troublesome in cylinder cleaners since the torque reaction causes the body of the cleaner to rock sharply as the motor starts. This can be at least a cause for user annoyance or even a source of danger to the user if the cleaner rolls over. With increasing suction requirements in cleaners, and hence more powerful motors, this is becoming a greater problem.
Similar problems occur in rotary floor cleaners where a rotating brush or mop is used to clean or polish a floor surface. The transient torque reaction produced can be annoying or even dangerous for the user because the machine can swing out of control when it is started.
Rudimentary forms of speed control for cleaning appliances have been available for many years and generally take the form of thyristors or triacs which are used to phase control the alternating supply voltage over a limited range. Typically these will allow the user to reduce the speed from 100% to some lower level, e.g. 70%. However, because of the inherent high starting torque of the motor, these forms of speed control are not particularly effective in controlling the starting transient.
One solution adopted for rotary floor cleaners has been to replace the series motor with a 3-phase induction motor driven by an inverter, e.g. as described by Kumaki in U.S. Pat. No. 4,992,718, which is incorporated herein by reference. In order to avoid a high starting current being drawn, Kumaki proposes a complex method to reduce the starting current for a given period of time using a microprocessor. While this proposal for reducing the starting current will have the incidental effect of reducing the starting torque, the system has no way to measure the speed and therefore no control over the speed at which the torque of the motor is varied.
There is therefore a need for a simple system capable of starting a floor cleaning appliance so that the transient torque is reduced to an acceptable level over the speed range from standstill to the working speed.